Air springs, also known as pneumatic springs, have been used in the suspension systems of motor vehicles, industrial machinery, and buildings for a number of years. An air spring comprises first and second end members, at least one of which is typically shaped like a piston, with an intervening flexible annular sleeve forming a sealed air chamber or cavity therebetween.
When compressive loads are applied to the end members, air pressure within the air chamber provides an opposing force which cushions rapid inward movements and supports steady loads. The annular sleeve typically deforms during movement in a manner guided by the piston-shaped end member(s). An air passage is usually provided between the interior and exterior of the air chamber for selectively adding or releasing air from the air chamber.
Air springs positioned between movable parts of the suspension system of a vehicle can serve to cushion shock loads impressed on the vehicle's suspension by road irregularities, thus improving the ride of the vehicle and reducing vibrational stresses on the vehicle structure, its passengers, and cargo.
Air springs can also serve as load leveling devices to maintain the proper body height and attitude of a vehicle subjected to various loading conditions. Proper vehicle body height can be important both when the vehicle is moving, e.g., to maintain bumper height, rear vision, and fender clearances, and when the vehicle is stationary, e.g., to maintain position at a loading dock during loading or unloading. One conventional measure of vehicle body height is the axle-to-body distance, i.e., the distance from the axle to a fixed reference point on the body. This measure must usually be kept within a permissible range of values to assure proper vehicle operation.
To manually control axle-to-body distance, it is known to provide an air source connected to the air chamber of the air spring and a valve which allows a user to selectively add or release air from the air chamber as desired to increase or decrease, respectively, the axle-to-body distance. Manual height control systems are relatively simple to implement, however, they cannot assure that a given axle-to-body height is maintained under service conditions of changing load, leaking air lines, etc.
To automatically maintain axle-to-body distance within a permissible range of values, a position sensor or height sensor is needed to monitor the positional relationship of a reference point on the frame with respect to the axle. The output of the height sensor is typically connected (possibly by means of a control circuit) to an air source valve to automatically add or release air to insure that the desired axle-to-body height is maintained.
It is known that a height sensor for use with an air spring can be located either externally or internally with respect to the sealed air chamber of the air spring itself. Locating the height sensor externally exposes the sensor to damage, corrosion and contamination from the external environment. Locating the height sensor inside the air cavity of an air spring protects the sensor from damage or contaminants from the external environment. However, the air cavity within an air spring has a limited amount of open space, is frequently occupied by moving structures such as bumpers, and the air itself may be a source of contaminants. These factors can complicate the internal use of conventional height sensors such as those utilizing mechanical, magnetic, or optical properties, to measure axle-to-body height in an air spring. U.S. Pat. No. 5,337,137 describes and air spring apparatus incorporating an optical path height sensor located within the air cavity. This height sensor is provided with a signal processor circuit which comprises a logarithmic converter circuit, a filter, and a rectifier circuit to overcome the undesirable effects of the contamination of the reflective surfaces within the air cavity. Although such a system may allow the use of an optical height sensor within the air cavity of an air spring, the complexity of the control system can make it more difficult and costly to integrate such a sensor into a given control system.
A need therefore exists, for a height sensor which can be incorporated into the air cavity of an air spring, yet which is simple to adapt into existing control systems. A need also exists, for an air spring incorporating a height sensor inside the air cavity, which height sensor being durable and simple to adapt into existing control systems.